In Exchange, external email flow refers to:

External email flow pertains to the process of sending and receiving emails that involve users located outside of the organization. This includes messages that originate from internal users and are sent to external recipients, as well as messages received from external sources into the organization's email system. Understanding this flow is crucial for IT administrators when managing email routing, security, and compliance.

The concept distinguishes external email from internal communications, which occur exclusively between users within the organization. This distinction is critical for implementing proper security measures, controlling spam filtering, and managing the overall email infrastructure effectively. Internal communications do not fall under external flow, while archived emails and spam are different considerations altogether, focusing on data management and security rather than the directionality of email traffic.